Breathing is a SKILL
I've written previously about breathing and how important it is in all endurance sports (and in life, really). In swimming, most athletes fail to exhale forcefully enough to get most of the air out of their lungs, reducing the amount they can take in on each breath.
For cycling, watch any great Time Trialist. You'll see the bloated belly extending and contracting with each breath as they utilize their abdomen to get as much oxygen as they can into their lungs.
There's a lot of content out there about relaxed breathing, offering specific techniques and postures in running and other sports. Most professional and elite athletes spend time learning how to maximize their breathing. After all, effective breathing is a skill, not an instinct.
I want you to consider a running breathing method that I've been using for five years now.
Rhythmic Breathing On An Odd Cadence
I first learned about this running breathing technique former Pro Triathlete and friend, Kevin Everett. It's called "rhythmic breathing on an odd cadence." For EASY or Endurance Runs, it's essentially inhaling for three strides and exhaling for two.
In other words, you'll switch from beginning to exhale on the right foot or the left foot every five steps.
For HARD Threshold efforts and above, inhale for two strides and exhale for one.
So for 3/2, it would be:
Inhale = right-foot strike, left-foot strike, right-foot strike (3)
Exhale = left-foot strike, right foot strike (2)
Switch your "lead" foot that you inhale on:
Inhale = LEFT foot strike, right-foot strike, left-foot strike (3)
Exhale = right-foot strike, left-foot strike (2)
When I first started trying this, I had a hard time adjusting my exhales to get more air out - especially when running up a hill. But, once I adjusted, I found I actually got MORE air in on my inhales. After only a month, I found that I felt stronger, more relaxed, and the effort felt easier.
Budd Coates talks about this breathing method in his YouTube video, called "Running on Air" (starting at 5:40). Go check it out and give it a try!
https://youtu.be/l7ucHjq-B6I "Running on Air" by Budd Coates